DC Basement ADU Requirements: Can Your Basement Qualify?
Not every basement can become a legal accessory apartment in Washington, DC. The District has seven big requirements that determine whether your basement qualifies. Here's what has to be true:
Your Zone Allows It
Most R-1, R-2, and R-3 zones let you build Accessory Apartments (ADUs) by right. That's huge—it means you don't need Board of Zoning Adjustment approval or a special exception. Just permits and inspections.
R-19 and R-20 zones (basically Georgetown)? Those require special exceptions, which makes everything harder, longer, and more expensive. If you're in Georgetown, we need to talk about what that process actually looks like.
You're Willing to Live on the Property
This is the rule that surprises everyone. DC requires owner-occupancy under Subtitle U §253. Either you live in the main house and rent the basement, or you live in the basement apartment and rent the main house.
What you cannot do: rent both units and live somewhere else. DC won't issue your rental license if you're not owner-occupying one of the units. This isn't negotiable, can't be waived, and the city actually checks. They want these apartments embedded in neighborhoods with owner accountability, not turned into absentee landlord situations.
The Apartment Stays Within Size Limits
Up to three (3) people may occupy the accessory apartment—except in R-19/R-20 zones, where the combined occupancy of your whole property (main house + accessory unit) can't exceed six (6) people total.
Size-wise: your accessory unit can't exceed 35% of your total house gross floor area (GFA). And your principal dwelling has to meet the minimum GFA for your zone (commonly 1,200 sq. ft. or 2,000 sq. ft., depending on your zoning district).
Your Entrance Complies
You can't add a new street-facing entrance for an accessory apartment—except in R-3, R-13, R-17, and R-20 zones, and even there, it must be below the main level and compatible with historic district guidelines if you're in one.
In R-1 and R-2 zones, no new street-facing entrance is allowed period. So we design compliant side or rear access instead. (This comes up constantly with row houses.)
Building Codes Are Met
Seven-foot ceilings minimum in living spaces per IRC R305.1. Code-compliant egress windows for bedrooms. Proper fire separation between your living space and the ADU. All the standard life-safety stuff applies.
A lot of older DC basements come up short on ceiling height. Sometimes you've got 6'2" or 6'6" of clearance—which means you need basement lowering to hit that 7-foot minimum. That's fixable but it costs money. (Only about 30% of the basements we evaluate actually need lowering, but when you need it, you need it.)
You'll Get the Rental License
Plan to rent it out? You need a Residential Rental Business License from the Department of Licensing and Consumer Protection before you can legally accept rent. There's an inspection involved. Registration with the city's Rental Accommodations Division.
It's not hard, but you can't skip it. The city will find out (trust us on this), and the penalties for unlicensed rentals are not worth it.
Our Basement ADU Conversion Process in DC
We've done enough of these to have a system. Five steps that take you from "Is this even possible?" to "Here are the keys to my new rental unit."
Step 1 — We Figure Out If Your Basement Actually Qualifies
Free in-home consultation at your property. We look at your zone, measure your basement, check existing conditions, and tell you straight up whether this makes sense.
What we're checking: Is your ceiling height workable or do you need underpinning or slab lowering? Are there moisture issues we need to address? Do your existing utilities have capacity for a second unit? How's your gross floor area looking relative to that 35% cap?
You'll know by the end of that visit whether you're moving forward and what the major cost drivers are going to be. No vague "we'll need to see" answers—we've done this enough times to know what we're looking at.
Step 2 — Design and Code Planning
This is where we solve the technical problems your basement has. (Because it's got problems. Every basement does.)
Ceiling height issues: A lot of DC basements sit at 5'10" to 6'6". Code wants 7 feet minimum for living spaces. Sometimes we can get there by lowering the floor in key areas. Sometimes you need full lowering, which means excavating under your foundation to create more headroom. That's expensive, but it transforms an unusable space into something people actually want to live in.
Egress windows: Bedrooms need windows you can escape through in an emergency. Specific size requirements, maximum sill height, window wells that meet code if you're below grade. We've installed hundreds of these throughout DC neighborhoods. They also happen to bring in natural light, which makes the space feel way less like a dungeon.
Moisture control: DC's clay soil and the fact that you're below grade means water management isn't optional. We design waterproofing systems based on your specific conditions—exterior drainage work if needed, interior barriers, proper grading.
Sound insulation: You don't want to hear your tenant's TV at midnight. They don't want to hear your kids at 6 AM. We build wall and ceiling assemblies that actually provide acoustic separation. (This matters more than people think.)
Separate utilities: Not required by code, but separate meters for electric, gas, and water make rent collection simpler and keep utility costs transparent. We'll design utility separation if you want it. What's more common: installing a mini-split so tenants can control their own temperature settings without affecting your HVAC bills.
Step 3 — We Navigate the DC Permit Process
DC permitting has improved over the years, but it's still not fun. You'll need a main building permit, then separate permits for electrical, plumbing, and mechanical work. Plan reviewers tear through submissions looking for code violations.
Good news: because we submit basement ADU permits regularly in DC, we know exactly what Department of Buildings reviewers flag. We build the plans right the first time, respond to comments quickly, and keep your project moving. We track everything through ProjectDox so you're never wondering what's happening.
Timeline? Usually 4-12 weeks for permits. Sometimes faster. Sometimes slower. (It's DC.)
Step 4 — Construction
This is the part where your basement stops being theoretical and starts being an apartment.
The scope varies wildly based on what your basement needs, but typically includes:
- Waterproofing and moisture control systems
- Egress window installation (cutting through foundation, installing windows, building code-compliant window wells)
- Fire-rated assemblies between your living space and the ADU
- Full electrical, plumbing, and HVAC installation
- Framing, insulation (including sound insulation), drywall
- Kitchen and bathroom installation
- Flooring, paint, trim, all the finish work
We coordinate inspections at each stage—basement lowering (if needed), framing, rough mechanical/electrical/plumbing, insulation, final. Miss something in an early inspection and you're tearing out work to fix it, which is exactly as expensive as it sounds.
Construction typically takes 2-4 months depending on scope. Simple conversions with decent existing conditions move faster. Need lowering or major structural work? You're looking at the longer end.
Step 5 — Final Inspection and Getting Your Rental License
Construction wraps with a final building inspection. Inspector walks the property, verifies everything matches approved plans, checks that all work meets code. Once you pass, you're done with the Department of Buildings.
But you're not done done.
If you're planning to rent this apartment, you need to get licensed through the Department of Licensing and Consumer Protection. This is separate from your building permit. You submit a Residential Rental Business License application (specifically for "Two-Family Rental"), schedule a housing inspection, and wait for approval.
The housing inspection checks for basic habitability: smoke detectors, carbon monoxide alarms, functioning heat, secure locks, no safety violations. Pretty straightforward if your construction was done right.
Final step: register with the Department of Housing and Community Development's Rental Accommodations Division. Then you're actually, legally done and can start collecting rent.
How Much Does a Basement ADU Cost in DC?
Nobody wants to hear "it depends," but... it depends.
About 70% of the basement ADU conversions we complete in Washington, DC cost somewhere between $100,000 to $250,000+. Where you land in that range comes down to what you're starting with and what needs fixing.
What Drives Costs Up
Your ceiling height situation matters more than anything else. If you've already got 7 feet of clearance, great—you just saved a ton of money. If you need lowering to create headroom, that's your single biggest expense. We're talking excavating under your foundation, shoring everything up, pouring new concrete. Specialized work. Expensive work. Only 30% of homes we evaluate need basement lowering, but when you need it, add $70,000 to $200,000 depending on size and complexities.
Egress windows run $9,000-$15,000 per window location. You're cutting through foundation walls, installing windows that meet specific size requirements, building exterior window wells with egress ladders or steps. Most basement ADUs need at least one, sometimes two.
Waterproofing scope depends on what's already going on with your basement. Light interior work costs $8,000-$12,000. Full exterior excavation and drainage systems can hit $25,000 or more. DC's clay soil doesn't mess around—you want this done right.
Separate utility systems—running new electric panels, gas lines, water meters—add $8,000-$18,000 depending on where your existing services are and how far we're running new lines. This isn't super common; most homeowners prefer to just account for utilities in the rent. What is common: installing a mini-split so tenants control their own temperature settings.
Your finish selections move the needle too. Builder-grade appliances and finishes keep costs reasonable. Start picking custom tile, high-end appliances, and premium fixtures? You'll blow past $150+ per square foot without trying.
Context on DC ADU Costs
Detached backyard ADUs—the ones where you're building a whole new structure from scratch—commonly cost $450,000 to $600,000 in DC according to established design-build firms. Basement conversions are almost always significantly cheaper because you've already got foundation, walls, and nearby utilities.
But "significantly cheaper" still means six figures for most projects. That's just reality in DC.
We don't do ballpark estimates without seeing your property. Too many variables. What we do is give you a detailed proposal that breaks out every cost—design, permits, construction, fixtures, contingencies. No surprises.
DC Basement ADU FAQ
Do I need a Certificate of Occupancy for a basement ADU?
Plan on it. DC requires a Certificate of Occupancy (C of O) for occupancy of all buildings except single-family dwellings. Once you add an accessory apartment and license the property as a Two-Family Rental, you're no longer in the single-family exception and should obtain a C of O via DOB's Certifi portal.
You'll also need a Basic Business License with a rental housing inspection and RAD registration.
What about parking—do I need to add spaces?
Usually not. Subtitle C §704.1 only requires new parking when an addition increases your building's gross floor area by 25% or more. Basement conversions work within your existing footprint, so they rarely trigger parking requirements.
Caveat: if your project converts a cellar (generally excluded from GFA calculations) into a basement (included in GFA) and materially increases your total GFA, we'll re-check parking applicability during design. But this is uncommon.
How does the owner-occupancy requirement actually work?
You must live in either the main house or the accessory apartment for the entire time the unit is rented. Most people live in the main house and rent the basement. Some live in the basement apartment and rent the main house.
What you cannot do: rent both units and live somewhere else. DC won't issue your rental license if you're not owner-occupying one of the units. This rule exists under Subtitle U §253 and can't be waived. The city actually enforces this.
How long does this whole thing take?
Design and permits typically take 4-12 weeks. (It's DC, so... anywhere in that range.) Construction runs 2-4 months depending on scope. Simple conversions with decent existing basement conditions move faster. Lowering or major structural work pushes you toward the long end.
Call it 4-9 months start to finish for most projects. Plan accordingly.
Can I use my basement ADU for an au-pair?
Absolutely. A code-compliant basement apartment is perfect for live-in childcare. Au-pair agencies and visa programs require "suitable private accommodations"—which means a real bedroom, bathroom, and living space where they can actually stand up straight, not a hunched-over cave with exposed ductwork.
Same code requirements apply (7-foot ceilings, egress windows, etc.), but you're not dealing with rental licensing since you're not charging rent. Just the building permits and inspections.
Why Choose Us for Your DC Basement ADU Project
We've Done This Before (A Lot)
Twenty years of basement ADU conversions in DC. Shaw, Dupont Circle, Adams Morgan, Capitol Hill, Chevy Chase, Columbia Heights, Georgetown, Kalorama, Palisades, Tenleytown, Foxhall—we know the city's quirks. We know which neighborhoods have challenging soil conditions, where water table issues pop up, and how long permits actually take with DC's Department of Buildings.
That local knowledge means fewer surprises and faster problem-solving when the inevitable curveball shows up. (There's always a curveball.)
Engineering & Inspections, All Coordinated
We work with licensed structural engineers and coordinate all third-party inspections from day one. No playing referee between parties who've never worked together before. No miscommunication about what the engineer actually meant.
Transparent Pricing
Our estimates break down design costs, permit fees, construction work, utility modifications, waterproofing systems, and finish options. No vague "allowances" that balloon later. We tell you what things actually cost upfront.
Licensed, Insured, Warranty-Backed
Fully licensed and insured in DC. We carry all required permits and licenses. And because we're doing the whole job—not just one piece of it—everything is warranty-backed.
The bottom line: Basement ADU conversions in DC are complicated enough. You shouldn't have to be the general contractor managing five different companies who've never worked together. Let us handle it.
Official Resources for Basement ADUs in DC
Quick Reference: Code Requirements for DC Basement ADUs
- Owner must live in either principal dwelling or accessory apartment
- Cannot rent both units and live elsewhere
- Required for entire rental period
Size & Occupancy Limits
- ADU cannot exceed 35% of total home GFA
- Up to 3 people in accessory apartment (most zones)
- R-19/R-20: Combined occupancy of principal + accessory cannot exceed 6 people
- Principal dwelling must meet minimum GFA for zone
- Habitable rooms (bedrooms, living): 7'-0" minimum
- Bathrooms: 6'-8" minimum
Egress Windows (IRC R310)
- Required for all bedrooms
- Max sill height: 44" above floor
- Min opening: 5.7 sq ft
- Window well: 9 sq ft minimum, 36" width
Entrance Requirements
- R-1, R-2: No new street-facing entrance allowed
- R-3, R-13, R-17, R-20: New entrance allowed below main level (with historic compatibility)
- Side or rear access complies in all zones
Rental Licensing
- Residential Rental Business License required from DLCP
- Housing inspection required
- RAD registration with DHCD
- Certificate of Occupancy required (Two-Family designation)
